1. Field of the Invention
The present invention relates to a thin film device including a thin film laminate structure such as a thin film transistor (hereinafter referred to as a TFT) and a method for making the same, and in particular relates to a thin film device capable of low cost production due to a decreased initial investment and a method for making the same. Also, the present invention relates to a liquid crystal panel and an electronic device using the thin film device.
2. Background Art
In recent years, liquid crystal display devices using such types of thin film devices have been used in notebook-type personal computers, car navigation systems, video cameras and various portable information devices, and their range of applications and production is drastically increasing. Such phenomena are due to improved performance including reduced price of the liquid crystal display devices, enlarged screen size, improved image resolution and low electrical power consumption. Further cost reduction is, however, required for further expansion of the market and range of applications.
The mainstream of the liquid crystal devices is active matrix liquid crystal devices using TFTs as switching elements for pixels. Each liquid crystal device includes TFTs, a TFT substrate on which a matrix of pixel electrodes connected to the TFTs are formed, a counter substrate provided with a common electrode, and a liquid crystal encapsulated between these two substrates. FIG. 17 shows the main section of a TFT substrate 60. In FIG. 17, TFTs 61 are formed at pixel positions near the intersections of a plurality of source or data signal lines S1, S2, . . . Sn arranged in columns with a plurality of gate or scanning signal lines G1, G2, . . . Gm arranged in rows. Source electrodes of the TFTs 61 are connected to their respective data lines, and drain electrodes are connected to their respective pixel electrodes 62. The data signal supplied from a data line is applied to a pixel electrode 62 through its corresponding TFT 61 based on the scanning timing signals supplied through the corresponding gate line. The state of the liquid crystal is changed and driven for displaying by an electric field between the pixel electrode 62 and the common electrode, not shown in the drawing.
The liquid crystal display device is fabricated by panel assembling including encapsulation of the liquid crystal between the TFT substrate 60 and the counter electrode, and packaging of driving circuits for driving the data lines and the gate lines. The cost of the liquid crystal display device greatly depends on the cost of the TFT substrate 60. The cost of the TFT substrate 60 depends on the manufacturing method of the TFTs. A part of driving circuits may be formed on the TFT substrate 60 by forming the active elements with the TFTs, and in this case, the cost of the TFT substrate represents a high proportion of the cost of the liquid crystal display device.
A TFT has a thin film monolithic structure including a plurality of thin films which include at least a silicon semiconductor layer having an insulating layer, a conductive layer, a source, a drain and a channel region. The cost of the TFT greatly depends on the production cost of the thin film monolithic structure.
The insulation layer in the thin film monolithic structure is formed by a low pressure chemical vapor deposition (LPCVD) process or a plasma enhanced CVD (PECVD) process, because a normal pressure CVD (NPCVD) process results in low uniformity of the film thickness. The conductive layer, or typically the metal layer, is formed by a sputtering process. The silicon film for forming the silicon semiconductor layer is also formed by the PECVD or LPCVD process. Further, a method for implanting an impurity into the silicon film by an ion implanting process or an ion doping process is used. Alternatively, the high concentration impurity region which functions as a source-drain region is formed of an impurity-doped silicon film in a CVD system.
The CVD systems and the sputtering system used in the above-mentioned film deposition processes belong to vacuum units for processing materials under vacuum pressures, and require large vacuum systems, resulting in an increase in initial investment. In the vacuum system, a substrate is transferred to a vacuum evacuation chamber, a substrate heating chamber, a film deposition chamber and a vent chamber, in that order, to form a film. The substrate atmosphere therefore must be changed from open air to vacuum, and this limits the throughput. Because the ion implanter and the ion-doping system are also vacuum systems, the same problems as above occur. Further, the ion implanter and the ion-doping system require complex mechanisms for generating plasma, extracting ions, mass-separating the ions (for the ion implanter), accelerating ions, collimating ions, scanning ions and so on, resulting in a remarkably high initial investment cost.
As described above, the thin film deposition technology and the processing technology for producing a thin film monolithic structure are basically similar to the manufacturing technology for LSI circuits. The main means for cost reduction of the TFT substrate include scaling-up of the substrate size for forming TFTs, improvement in efficiency of the thin film deposition and its processing step, and improvement in yield.
Scaling-up of the substrate size for producing large liquid crystal display devices with reduced costs is an obstacle to high speed transfer of the substrates in the vacuum system, and causes breakage of the substrate due to thermal stress during the deposition steps, hence it is significantly difficult to improve the throughput of the film deposition system. Also, the scaling-up of the substrate size inevitably requires scaling-up of the film deposition system. An increased cost accompanied by the increased volume in the vacuum system further increases the initial investment, and as a result, it is difficult to achieve drastic cost reduction.
Although an increased yield is a valuable means for cost reduction, a yield near the limit has been achieved, and thus drastic cost reduction is difficult in view of the yield.
Patterning of each layer is performed by a photolithographic process. The photolithographic process essentially includes a coating step, an exposure step and a developing step of a resist film. After these steps, an etching step and a resist-removing step are required, hence the steps for patterning is a factor in increasing the number of steps for thin film deposition. This is a factor in the increased cost of thin film device production.
Regarding the resist-coating step in the photolithographic process, only less than 1% of the resist solution dropped onto the substrate remains on the substrate as the resist film after spin coating, reducing the efficiency of the use of the resist solution.
Although a printing process has been proposed as a low cost process instead of a large scale exposure system used in the exposure step, it has not yet reached practical use due to problems such as processing accuracy.
As described above, it is not possible to drastically reduce the cost of the TFT substrate, although the market requires drastic price reduction of the liquid crystal display devices.
It is an object of the present invention to provide a thin film device and a method for making the same, in which a part, or all of, the films in a thin film monolithic structure used for a liquid crystal display device are deposited without a vacuum system in order to decrease initial investment and operation costs, increase the throughput and significantly decrease the production costs.
It is another object of the present invention to provide a thin film device and a method for making the same, in which a thin film having characteristics similar to those of a CVD or sputtered film is formed of a coating film while achieving cost reduction.
It is a further object of the present invention to provide a thin film device and a method for making the same, in which the consumption of a coating solution is decreased in the formation of the thin coating film for achieving cost reduction.
It is still another object of the present invention to provide a thin film device and a method for making the same, which is capable of patterning the formed film without a photolithographic process and, thus, reducing the cost.
It is a still further object of the present invention to provide a thin film device, a liquid crystal panel and an electronic device using the same, in which a plane in contact with the liquid crystal can be planarized by forming a pixel electrode with a coating film.
It is another object of the present invention to provide a thin film device, a liquid crystal panel, and an electronic device using the same, in which a wiring layer can be used as a light-shielding layer for a black matrix and the thin film device has a high aperture ratio.
It is still another object of the present invention to provide a liquid crystal panel and an electronic device which enable cost reduction due to use of an inexpensive thin film device.